1. Field of the Invention
This invention relates generally to image transfer technology and more specifically to electrophotography. The invention is a positive charging, organic photoconductor material with superior surface release characteristics for dry and liquid toner electrophotography.
2. Related Art
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity, but different levels of charge.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers melts and fixes the toner in the paper, subsequent to direct transfer, or indirect transfer when using an intermediate transfer medium, producing the printed image.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered, the basic printing colors --yellow, magenta, cyan and black, may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multicolored image.
The important photoconductor surface, therefore, has been the subject of much research and development in the electrophotography art. A large number of photoconductor materials have been disclosed as being suitable for the electrophotographic photoconductor surface. For example, inorganic compounds such as amorphous silica (SiO.sub.2), arsenic selenite (As.sub.2 Se.sub.3), cadmium sulfide (CdS), selenium (Se), titanium oxide (TiO.sub.2) and zinc oxide (ZnO) function as photoconductors. However, these inorganic materials do not satisfy modern requirements in the electro-photography art of low production costs, high-speed response to laser diode or other light-emitting-diode (LED) and safety from non-toxicity.
Therefore, recent progress in the electrophotoqraphy art with the photoconductor surface has been made with organic materials as organic photoconductors (OPC). Typically, the OPC's in the current market are of the negative charging type with a thin charge generation material layer beneath a thicker charge transport material layer deposited on top of the charge generation layer. The negative charging OPC's perform well for xerographic copiers and printers in the following applications:
a. Low end (4-10 copies per minute) and high end (more than 50 copies per minute) xerographic systems using dry powder developers of one or two colors, or using liquid developers for black and white copies only; and,
b. High image quality (above 1800 DPI) color proofing, lithographic plate printing and master xero-printing systems with life expectancies of less than 100 cycles.
However, prior art negative charging OPC's also have several drawbacks, namely:
1. Large amounts of ozone are generated in the negative corona charging process, creating environmental concerns. This problem has been addressed by installing ozone absorbers like activated carbon filters, and by using contact negative charging instead of corona charging. These ozone remediation approaches, however, have drawbacks of their own and are not attractive commercial solutions.
2. Negative corona charging generally results in less charge pattern uniformity compared to positive corona charging. Lower charge pattern uniformity in turn results in more noise and less definition in the final image.
3. In small particle toner processes, including fine dry powder and liquid toner processes, designers have been able to develop more charge stability in positively charged toners than in negatively charged toners. Therefore, positive charging OPC's are preferred for a discharged area developed image as in laser printers.
Specific morphologies of phthalocyanine pigment powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate. In these phthalocyanine/binder photoconductors, the photo-generation of charge and the charge transport occur in the particles of the phthalocyanine pigment while the binder is inert. Therefore, the photoconductor may be made of a single layer of phthalocyanine/binder. These single-layer photoconductors are known to be very good positive charging OPC's due to the hole (positive charge) transportability of the phthalocyanine pigment.
In these single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 10-30 wt. %, high enough to perform both charge generation and charge transport functions, with the binder content being in the range of about 90-70 wt. %. The single photoconductor layer is usually more than about 3 microns (um) thick in order to achieve the required charge acceptance and resulting image contrast.
Also, it is known to use phthalocyanine pigment as a charge generation component in a multi-layer photoconductor. Today, the commercially available OPC for digital electrophotography, wherein the writing head is LED array or laser diode, uses such a multi-layer photoconductor. The charge generation layer containing the phthalocyanine pigment is usually less than 1 micron (um) thick. A charge transport layer about 20-30 microns (um) thick and containing transport molecules other than the phthalocyanine pigment, is overcoated on top of the charge generation layer.
These types of multi-layer OPC's, however, are only used as negative charging ones, so they have all the drawbacks of negative charging OPC's discussed above. So, there remains a strong incentive for the development of a phthalocyanine pigment positive charging OPC.
One response by the industry to this incentive has been to investigate a positive-charging, multi-layer OPC with an electron transport molecule in the upper layer which must be an electron acceptor molecule and an electron transporter molecule under the application of a positive electric field. See, for example, the disclosure of U.S. Pat. No. 4,559,287 (McAneney, et al.). These types of OPC's use derivatives of fluorenylidene methane, for example, as the electron acceptor and transport molecule. These types of molecules, however, exhibit poor solubility, resulting in recrystallization in the OPC forming mixture during coating, poor compatibility with popular binders, and poor reaction yield resulting in high production costs. Also, these types of molecules tend to be highly carcinogenic, resulting in safety risks to workers and users and therefore, low market receptivity.
Also, U.S. Pat. No. 5,087,540 (Murakami et al.) discloses a positive charging, single-layer photoconductor for electrophotography which has X-type and/or T-type phthalocyanine compound dispersed partly in a molecular state and partly in a particulate state in a binder resin. To make the dispersion, the phthalocyanine compound is agitated in a solvent with the binder resin for from several hours to several days. This approach, therefore, has manufacturing drawbacks.
Another response by the industry to the incentive for the development of a phthalocyanine type positive charging OPC has been to investigate a multi-layer OPC wherein the relative positions of the charge generation and transport layers are reversed. See, for example, the disclosure of U.S. Pat. No. 4,891,288 (Fujimaki et al.). These types of OPC's, however, require a protective overcoat to avoid mechanical damage to the OPC because the upper pigment-containing layer is very vulnerable to the development component, the transfer medium component and the cleaning component in the electrophotographic system. These overcoat layers have problems of their own, increasing the residual voltage of the photoconductor and increasing its electrical instability. See, for example, the disclosures of U.S. Pat. Nos. 4,923,775 (Schank) and 5,069,993 (Robinette, et al.).
Therefore, it is an object of this invention to provide a phthalocyanine type positive-charging OPC which exhibits stable electrical properties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity electrophotographic process. Modern digital imaging systems wherein the writing head is LED array or laser diode, have very high light intensities (about 100 ergs/cm.sup.2) over very short exposure time spans (less than 50 nano seconds), resulting in severe conditions for the OPC compared to optical input copiers with light intensities between about 10-30 ergs/cm.sup.2 and exposure times between about several hundred micro-seconds to mili-seconds.
Unfortunately, there is no product on the market today which provides such stable electrical properties. This is because the phthalocyanine type positive-charging OPC exhibits instability when it is frequently exposed to the corona charger and the intense light source in the electrophotographic process. I have discovered this instability to be more pronounced at the strong absorption, high light intensity, short exposure time conditions required for the laser printing process. The instability is exhibited in the significant increase of the dark decay after a small number of repeat cycles of laser printing. Also, the instability is exhibited in the decrease in surface potential. These instabilities cause deleterious changes in image contrast, and raise the issue of the reliability of image quality.
Also, I have discovered that these instabilities in the phthalocyanine/binder photoconductor seem to be independent of the chemical structure or morphology of the pigment. Instead, they appear to be dependent on the nature of the contact between individual pigment particles. These observations of mine have been made only recently, and there is no report or suggestion in the prior art about how to effectively address and solve the problem of photoconductor instability in the high cycle, high severity electrophotographic process.
Preferably, desirable electrophotographic performance may be defined as high charge acceptance of about 30-100 V/um.sup.2, low dark decay of less than about 5 V/sec., and photodischarge of at least 70% of surface charge with the laser diode beam of 780 nm or 830 nm frequency, through the optical system including beam scanner and focus lenses, synchronized at 0.05 micro seconds for each beam.
When conventional binders for the phthalocyanine pigment, such as acrylic resins, phenoxy resins, vinyl polymers including polyvinylacetate and polyvinyl butyryl, polystyrene, polyesters, polyamides, polyimides, polycarbonates, methylmethacrylates, polyurethanes, polyureas, melamine resins, polysulfones, polyarylates, diallylphthalate resins, polyethylenes and halogenated polymers, including polyvinylchloride, polyfluorocarbon, etc., are used, acceptable charge acceptance and photodischarge are obtained, provided a good dispersion of the pigment in the binder is obtained. However, among these polymers which result in good performance for charge acceptance and photodischarge, none of them exhibit the desirable stability under the LED array or laser diode exposure conditions. Also, any binders, and accompanying solvents, which do not form a stable dispersion with the phthalocyanine pigment usually exhibit very slow charge acceptance, high residual voltage, or high dark decay, and are therefore unacceptable.
Another important object of the present invention is to provide a positive-charging OPC having superior surface release characteristics. In the context of this invention, superior surface release characteristics means that the photoconductor surface has low adhesion which permits easier transfer of the toner particles image off the photoconductor surface onto the plain paper or intermediate transfer medium. The current electrophotography requires the plain paper as the final medium for the image, i.e. the toner image on the photoreceptor must be well transferred to the plain paper by known arts such as electrostatic charge or non-electrostatic thermally assisted transfer. The high transfer efficiency toning systems have the benefit of high image density on the plain paper, with the high image quality being due to a completely transferred image which results in reduced efforts for cleaning the photoreceptor surface. The requirement of superior surface release characteristics also is crucial for high speed printing systems, especially for small particle developers such as dry microtoner (particle size less than 5um) and liquid toner (particle size in the submicron range).
In the last decade, there have been a lot of efforts to enhance image transfer efficiency in the electrophotographic systems, such as release surface coated toner, intermediate transfer concepts and systems and temporary release coating on the surface of the photoconductor.
Even so, the image transfer problems have not been completely solved as the above-proposed solutions give rise to other problems. For example, higher cost and reduced printing speed are encountered with the intermediate transfer approach. Also, the release surface coated toner technologies encounter the difficulty of controlling particle size and poor fusing effect as the release coating materials are highly crosslinking polymers. Also, the temporary release coating of the photoreceptor approach is not a suitable one from the service-free perspective.
The photoconductor of this invention aims at a solution for a permanent, reusable organic photoconductor having superior surface release characteristics, and therefore, high efficiency toner particle transfer. This approach is found to be very effective in the simplification of the plain paper imaging process at low cost.